Piezoelectric materials composed of lead zirconate titanate (PZT) and the like have been conventionally used as mechanoelectrical conversion elements such as driver elements and sensors. In addition, in order to respond to demands for reduced device size, higher density and reduced costs, there has recently been an increase in the use of mechanoelectrical conversion elements based on micro electromechanical systems (MEMS) using silicon substrates.
It is preferable to make piezoelectric materials with a form of thin film when composing mechanoelectrical conversion elements with MEMS. As a result, high-precision processing has become possible that uses deposition, photolithography and other semiconductor process technology, thereby making it possible to reduce the size and increase the density of mechanoelectrical conversion elements. In addition, costs can be reduced since configuration of mechanoelectrical conversion elements with MEMS allows a plurality of mechanoelectrical conversion elements to be processed collectively using large area wafers. Moreover, the conversion efficiency of mechanoelectrical conversion elements can be improved, and the characteristics of driver elements as well as sensor sensitivity can also be improved.
Known examples of methods used to deposit PZT and other piezoelectric materials on a silicon (Si) substrate, for example, include chemical methods such as CVD, physical methods such as sputtering or ion plating, and methods such as sol gel methods involving the growth of piezoelectric materials using a liquid phase.
Piezoelectric materials such as PZT are able to realize favorable piezoelectric effects when the crystal structure thereof adopts a perovskite structure. FIG. 6 is a drawing indicating the crystal structure of PZT having a perovskite structure. As shown in FIG. 6, PZT is composed of a mixture of lead titanate and lead zirconate, and can be seen to have a perovskite structure in which titanium (Ti) or zirconium (Zr) is located in the center of a cube, lead (Pb) is positioned at each of the apices, and oxygen is positioned at the center of each side.
In addition, thin films of piezoelectric materials having a perovskite structure allow the obtaining of large piezoelectric characteristics when the thin film adopts a homogeneous single crystal structure.
This tendency is known to be prominent in substances referred to as relaxer materials such as lead magnesium niobate (PMN) or lead zinc niobate (PZN) in which the zirconium (Zr) or titanium (Ti) of PZT is substituted with another element (FIG. 4 on p. 29 of Non-Patent Document 1).
However, since piezoelectric materials such as PZT and Si have different lattice constants, when a piezoelectric material is deposited on an Si substrate, the piezoelectric material adopts a polycrystalline structure in which a plurality of crystals having different orientations are gathered together in the form of columns as shown in FIG. 7 (FIG. 9 on p. 133 of Non-Patent Document 1). FIG. 7 is a cross-sectional view of a piezoelectric material when a piezoelectric material composed of PZT has been formed on the upper surface of a substrate. In the piezoelectric material shown in FIG. 7, although crystal orientation is aligned in a single region, the crystal orientations of adjacent regions differ, and the piezoelectric material has a polycrystalline structure. In the case of a polycrystalline structure, restriction of displacement occurs at the crystal grain boundaries, and piezoelectric characteristics decrease in comparison with a single crystal structure due to the effects thereof. In addition, there is also the problem of being unable to apply a large electric field due to current leakage attributable to the crystal grain boundaries in the case of a polycrystalline structure.
The following technologies are known for solving such problems. Patent Document 1 discloses a technology consisting of providing a relaxing layer composed of MgO and the like between a piezoelectric material and an Si substrate in order to alleviate the mismatch in lattice constants between the two.
In addition, Patent Document 2 discloses a method for single crystallization of a ferroelectric thin film in a method for forming a ferroelectric thin film on a substrate by sputtering, wherein by using a substrate in the shape of a strip, a prominent difference is made to occur in tensile stress or compressive stress between the long sides and short sides of the substrate during cooling after sputtering.
In addition, Patent Document 3 discloses a technology for single crystallization of a piezoelectric material by forming two lower electrodes on a substrate, depositing a piezoelectric material thereon, removing superfluous regions of the two lower electrodes and piezoelectric material so that the two lower electrodes and the piezoelectric material are formed on the substrate in the form of a plurality of columns, and subsequently subjecting to heat treatment.
However, in the technology of Patent Document 1, although a relaxing layer is provided between a piezoelectric material and an Si substrate, due to the large difference in crystal constant between the piezoelectric material and the Si substrate, there is the problem of the piezoelectric material not undergoing single crystallization when the piezoelectric material is formed over a wide range.
In addition, in the technology of Patent Document 2, since a relaxing layer is not provided between a substrate and a ferroelectric thin film resulting in a large mismatch in the lattice constants between the two, there are limits on single crystallization of the ferroelectric thin film.
In addition, in the technology of Patent Document 3, although a piezoelectric material is subjected to single crystallization by heat treatment, since a relaxing layer is not provided for alleviating the difference in lattice constant between the lower electrodes and a piezoelectric material, there are certain limits on single crystallization of the piezoelectric material. In other words, in the case of heat treatment following deposition, since there are limitations on the range of atomic movement, in cases in which there are large variations in the crystal orientation of the piezoelectric material, single crystallization of the piezoelectric material is difficult even if subjected to heat treatment.
Patent Document 1: Japanese Patent Application Laid-open No. H5-139892
Patent Document 2: Japanese Patent Application Laid-open No. H5-235428
Patent Document 3: Japanese Patent Application Laid-open No. H6-215975
Non-Patent Document 1: “High-performance piezoelectric materials and advanced application technologies”, Science & Technology Co., Ltd.